With the forthcoming entry into force of the Euro IV standard on vehicle exhaust emissions, NOx (or nitrogen oxide) pollution control devices must be installed.
The system selected by most automobile manufacturers for reducing NOx emissions to the requisite level generally consists in implementing a selective catalytic reaction with reducing agents such as urea (“Urea SCR” or Selective Catalytic Reduction using the ammonia generated in situ in the exhaust gases by the decomposition of urea).
For this purpose, it is necessary to equip the vehicles with a tank containing a urea solution, and a device for batching the quantity of urea to be injected into the exhaust line. Since the aqueous urea solution generally used for this purpose (eutectic with 32.5% by weight of urea) freezes at −11° C., it is necessary to provide a heating device to liquefy the solution in order to inject it into the exhaust line for starting in freezing conditions. Such a device is the subject of a co-pending application in the name of the applicant.
Furthermore, to facilitate the operation and restarting in the case of freezing, it is advantageous periodically to purge the lines that convey the urea in order to clear them of the liquid deposits of additives, which are liable to solidify in case of frost. In particular, it is advantageous to purge the line connecting the additive tank to the injector (or injection line). This also serves to avoid using a complex device for heating the urea lines.
Several systems have been provided for this purpose in the prior art.
Thus, U.S. Pat. No. 5,884,475 describes a system in which the injection line is subjected to consecutive purges respectively in the portion close to the additive tank and in the portion close to the injector. It uses a set of complicated and costly valves for this purpose.
Application WO 03/033111 describes a purge system that is simpler in terms of control/valves, but which has the drawback of bypassing the pump (and the neighbouring line portions) present on the injection line. Such a system is therefore less reliable.